Petroleum is a mineral oil formed by a mixture of organic compounds trapped in very diverse geological formations. Thus, each petroleum deposit holds a specific quality of petroleum, determined by the relative proportions of the different organic compounds, including biomarkers, of which it is constituted.
These organic compounds are essentially hydrocarbons, including saturated compounds: the n-alkanes, iso-alkanes and cycloalkanes, aromatic compounds, resins or also asphaltenes.
Among the cycloalkanes, the diamondoids, tricyclic terpanes, hopanes, steranes may be mentioned. Among the iso-alkanes, pristane, phytane may be mentioned.
The diamondoids are 3-dimensional polycyclic saturated organic compounds which are in the form of a cage and can be substituted, or unsubstituted, by alkyl groups. Among these diamondoids, adamantane (C10H16 compound), diamantane (C14H20) or also triamantane (C18H24) may be mentioned non limitatively, as well as their homologues comprising at least one alkyl branch.
These diamondoids are therefore natural constituents of petroleum, which is also called “oil” in the remainder of the present description. The diamondoids are commonly found in oils at concentrations greater than 1 ppm.
Because of their unique physico-chemical properties (high thermal stability, high melting and vapour pressure points), there has for several years been growing interest in diamondoids in a large number of fields (pharmaceuticals industry, medicine, nanotechnology, micro-electronics, etc.) including the petroleum sector.
Moreover, due to their high stability in oils, it is beneficial to carry out analyses, both qualitative and quantitative, of these diamondoids in order to have a better understanding of the petroleum systems, in particular of biodegraded and cracked oils. Such analyses make it possible in particular to evaluate the geological maturity of an oil field and/or the level of thermal maturity of oils, to distinguish between two oils and/or to characterize mixtures of oils, to evaluate the degree of advancement of the biodegradation of the oils, to determine the oil/oil or mother rock/oil correlations. The usefulness of the biomarkers present in the oils, and of the diamondoids in particular, for carrying out these analyses, explains the increase in the number of studies carried out aiming at separating and/or concentrating and/or identifying the iso-alkanes and the cycloalkanes in general, and the diamondoids, in particular.
Such qualitative and/or quantitative analyses are in general carried out by gas chromatography (GC) or by gas chromatography coupled with mass spectrometry (GC/MS). As for the isotopic analyses, they are carried out by gas chromatography coupled with isotope ratio mass spectrometry (GC/irMS).
However, in view of the wide variety of compounds forming the oil and the very small quantity of these compounds of interest within an oil, it proves necessary to prepare the sample of oil before carrying out its chromatographic analysis (by GC, GC/MS or GC/irMS) in order to purify, to isolate and/or to concentrate the specific compounds that are to be studied.
Thus, methods for separating fractions of iso-alkanes and cycloalkanes which comprise several steps are described in the literature.
Reference can in particular be made to the scientific publication by L. Huang et al. (“A novel method for isolation of diamondoids from crude oils for compound-specific isotope analysis”—Organic Geochemistry—42 (2011) p.566-571) which describes a three-step method:                a first separation step by liquid chromatography through a column comprising an activated silica gel in order to collect a saturated fraction of hydrocarbons comprising the n-alkanes and the cyclic and branched hydrocarbons        a step of concentration under nitrogen of this saturated fraction of hydrocarbons,        a second separation step of this concentrated saturated fraction of hydrocarbons by liquid chromatography through a column comprising a molecular sieve (ZSM-5 zeolite of silicalite type) in order to collect the fraction of cyclic and branched hydrocarbons.        
Reference can also be made to the scientific publication by M. Audino et al. (“Macrocyclic alkanes in crude oils and sediment extracts: enrichment using molecular sieves”—Organic Geochemistry—35 (2004) p. 661-663) which describes a method comprising at least two successive steps of chromatographic separation on columns, each columns comprising a defined molecular sieve, it being noted that an intermediate filtration step followed by an intermediate evaporation step is necessary between each chromatographic separation step.
It is observed that the methods that have just been described comprise at least two separation steps by liquid chromatography in order to obtain as a result the sought fraction of iso-alkanes and cycloalkanes. Moreover, the methods described in these publications also comprise at least one intermediate evaporation step of the intermediate fractions collected before the second separation step, this intermediate evaporation step being optionally preceded by a filtration step, as in the publication by M. Audino et al.
However, the increase in the number of steps necessarily increases the risks of loss and/or contamination of the samples. More particularly, the intermediate evaporation step of the collected fractions can lead to the more or less partial loss of the lightest compounds such as the adamantanes.
The aim of the present invention is therefore to provide a method for separating by liquid chromatography a fraction of iso-alkanes and cycloalkanes from a sample of a mixture of organic compounds at least partially overcoming the aforementioned drawbacks.
More particularly, the invention relates to a separation method which allows the isolation, from a sample of such a mixture of organic compounds, of a fraction of iso-alkanes and cycloalkanes (containing in particular the diamondoids) in a minimum number of steps, without the risk of contamination of the samples and/or the loss of compounds, in particular the lightest compounds, caused by the intermediate evaporation and optional filtration steps of the methods of the prior art.